feat(plot): page setup manager — named setups, layouts, <none>/<previous>

Restructure the plot dialog into a style-manager-style master/detail: a
left list of page setups plus a right settings editor. The list shows
<none> (defaults + PDF) and <previous> (the settings the dialog opened
with), then the drawing's layouts (*Model*, *Layout1*, …) and its
standalone named page setups.

- Named page setups live in the ACAD_PLOTSETTINGS dictionary
  (src/scene/page_setup.rs: names/get/save/delete/rename, creating the
  dictionary if absent); DWG round-trip verified.
- Toolbar New / Copy / Delete | Set current / Preview / Print. Selecting
  a row loads it into the editor; Set current writes to the active
  layout. Copy duplicates a layout or a setup into a new named setup;
  Delete/Rename apply only to named setups.
- Scene::plot_settings_for(name) generalizes effective_plot_settings so
  any layout's settings can be loaded; on open the settings are
  snapshotted for <previous>.
- Default selection: model space → <none>; a layout → its own setup.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/mod.rs

@@ -521,6 +521,9 @@ pub(super) struct OpenCADStudio {
     plot_orientation: crate::io::paper_sizes::Orientation,
     /// Backing state for the full Plot / Print dialog.
     plot_dialog: crate::ui::window::plot::PlotDialogState,
+    /// Snapshot of the dialog's settings taken when it opened, restored by the
+    /// `<previous>` list entry.
+    plot_prev: Option<crate::ui::window::plot::PlotDialogState>,
 
     // ── Plot Style Table ──────────────────────────────────────────────────
     /// Currently loaded CTB/STB table (None = no override).
@@ -2140,6 +2143,7 @@ impl OpenCADStudio {
             plot_format: crate::io::paper_sizes::PaperSize::A4,
             plot_orientation: crate::io::paper_sizes::Orientation::Landscape,
             plot_dialog: crate::ui::window::plot::PlotDialogState::load(),
+            plot_prev: None,
             opening: None,
             pending_close: None,
             save_dialog_format: "DWG 2018".to_string(),